Nonvolatile memory integrated circuit devices can maintain stored data even if the power is turned off. Thus, the nonvolatile memory integrated circuit devices are widely used in information communication devices such as digital cameras, cellular phones, personal digital assistants (PDA), MP3 players and many other applications. The nonvolatile memory integrated circuit devices should have large capacity and high integration density in order to be used in high-function and multi-function information communication devices. There is an increasing demand for high integration of memory cells of the nonvolatile memory integrated circuit devices and lower cost per Mbyte. To meet this demand, a multi-bit (e.g., 2-bit) nonvolatile memory cell has been developed.
In a conventional 2-bit nonvolatile memory cell, four threshold voltage levels are set and the set threshold voltage levels indicate different states. However, as the gate line width of the 2-bit nonvolatile memory cell decreases, due to increased integration density, short channel effects may make it difficult to distinguish threshold voltages. Moreover, since the influence of hot carriers may increase due to a short channel, electrons may be gradually trapped in a tunneling layer. As a result, larger numbers of defective devices may be manufactured and the non-defective devices may have low quality. An increase in an operating voltage for the purpose of enlarging a program acceptance window may not be desirable because it may reduce device reliability and/or may increase the interference between memory cells.